I wonder why people select those payment dates. Maybe a lot of people get paid in the middle of the month?
My calendar also visually shows that as well (just past middle point of month is biggest payment week).
Image: http://i.imgur.com/BMB5bna.png

Yes, when there are holidays on Monday, we get the big payout on Friday and it is a combined payment of loans that were due on on the previous Saturday, Sunday, Monday and Tuesday. A Friday holiday causes a larger payout on Thursday as you get loans that were due on the previous Friday, Saturday, Sunday and Monday.
What happens to loans with payment dates of the 31st? Do these get collected on the 30th in short months, or on the following 1st? The end of February should generate a large payment as well since loans that come due on 29th, 30th and 31st will either get collected on the 28th or on the 1st of March. (EDIT: Just looked back and the 28th of Feb and 1st of March fell over a Saturday/Sunday this year....hence why your first Thursday in March payment was larger than the one later in the month)